This invention relates to ink jet head arrangements and, more particularly, to a new and improved ink jet head arrangement having a simple and inexpensive structure.
Conventional ink jet heads, in which ink received from an ink reservoir is ejected selectively through a series of orifices, have been made using thin plates of metal or ceramic material having appropriate passages which are bonded together in adjacent relation in an assembly, as described, for example, in the Roy et al. U.S. Pat. No. 5,087,930 and the Hoisington et al. U.S. Pat. No. 4,835,554. In such arrangements, each chamber or passage in the flowpath leading from the ink inlet to the orifice, through which the ink is ultimately ejected, is provided in one or more of the several plates in the assembly. This requires an array of plates having different thicknesses, each of which must be separately machined to precise dimensions to produce the appropriate chambers and passages, and also requires precise positioning of all of the chambers and passages in the plates. Moreover, the plates must be assembled and bonded together and to a piezoelectric plate in highly precise alignment, and each plate must be flat and free from burrs that would cause voids between adjacent plates. Furthermore, because of differences in the coefficients of thermal expansion between the materials used in the plates, bond stresses are generated by temperature variations which occur in connection with the manufacture and use of the ink jet head which must be overcome.
In hot melt ink jet printheads, which operate at elevated temperatures, the printhead materials must be good conductors of heat so that the printhead will warm up quickly and the temperature gradients during operation will be small. The stresses created when parts of different materials expand differently with changes in temperature is another concern. The prime mover in a printhead is usually a piezoelectric ceramic (PZT) which has a relatively low thermal expansion coefficient. For optimum printhead design, the challenge is to find other materials which are close to this expansion. If the printhead materials cannot be matched, it is desirable to have low-modulus materials to reduce the stresses.
The ink passages in an ink jet printhead are fine features with tight tolerances. To maintain such tight tolerances, the manufacture of the printhead requires low machining forces, small tool deflection and small machining errors, no plastic deformation and no burrs. Moreover, it may be desirable, particularly in development, that the manufacture should be carried out using standard machining methods, such as grinding, milling, drilling and shaping.
In addition, the printhead should be made of materials which are chemically inert and do not change shape over time when loaded or oxidize or interact with organic chemicals found in hot-melt and other inks or with pigments or dyes in the inks.
Heretofore, some plates used in ink jet heads have been photo-etched to provide the appropriate chambers and passages, which has the advantage that the plates are generally burr-free and can be made from Kovar material, stainless steel and other materials that have appropriate mechanical and thermal expansion characteristics. The materials useful for photoetching, however, have drawbacks when used in connection with ink jet heads from which hot melt ink is ejected since they generally have low thermal conductivity. In addition, the photo-etching process has the disadvantage of being a batch process with lot-to-lot variations and, moreover, when used in this manner, produces a relatively large quantity of chemical waste.
Furthermore, conventional piezoelectric plates used in ink jet heads are thin, fragile and susceptible to damage during processing. Because of the greater likelihood of damage to larger plates, the maximum size of piezoelectric plates is normally quite small, for example, less than about 100 mm, which correspondingly limits the length of an array of orifices through which ink is ejected as a result of the actuation of the piezoelectric plate.